Description will be made by using as an example a fixing device (heating device) in an image forming apparatus such as an electrophotographic copier, a printer or the like. A fixing device for use in image forming apparatuses is a device that permanently fixes an unfixed toner image on a surface of a recording material by heat. The unfixed toner image has been formed on the recording material using toner formed from a thermally meltable resin or the like. The fixing of the toner image is performed by an appropriate image formation processing method such as electrophotography, electrostatic recording or the like.
As a method used most commonly for such fixing devices, a roller fixing method has been known. In the roller fixing method, a recording material is introduced into a nip part formed by a heating roller that is heated and adjusted so that a predetermined fixing temperature is attained and a pressing roller that is opposed to and is in contact under pressure with the heating roller. At the nip part, the recording material is conveyed while being sandwiched between the rollers so that an unfixed toner image is fixed on a surface of the recording material by heating. As a heat source of a heating roller for use in the roller fixing method, a halogen lamp has been in frequent use.
Meanwhile, in recent years, in response to the demand for a reduction in power consumption and warm-up time, roller heating type devices employing an electromagnetic induction heating method have been proposed. FIG. 11 shows an example of a conventional induction heating fixing device including a heat generating roller that is heated by electromagnetic induction (see, for example, JP11(1999)-288190 A).
In FIG. 11, reference numeral 820 denotes a heat generating roller including a core material 824 made of metal, an elastic layer 823 that is formed from a heat-resistant foam rubber and molded integrally on an outer side of the core material 824, a heat generating layer 821 formed of a metallic tube, and a mold releasing layer 822 provided on an outer side of the heat generating layer 821, which are provided outwardly in this order. Reference numeral 827 denotes a pressing roller that is formed from a heat-resistant resin and has the shape of a hollow cylinder. A ferrite core 826 wound with an excitation coil 825 is placed in an inner portion of the pressing roller 827. The ferrite core 826 applies pressure to the heat generating roller 820 through the pressing roller 827, and thus a nip part 829 is formed. While the heat generating roller 820 and the pressing roller 827 rotate in the respective directions indicated by arrows, a high-frequency current is fed through the excitation coil 825. This causes alternating magnetic fields H to be generated, so that the heat generating layer 821 of the heat generating roller 820 is heated rapidly by electromagnetic induction to a predetermined temperature. While predetermined heating is continued in this state, a recording material 840 is inserted into and passed through the nip part 829. Thus, toner images 842 formed on the recording material 840 are fixed on the recording material 840.
Furthermore, in addition to devices of the above-mentioned roller heating type using the heat generating roller 820 having the induction heat generating layer 821 as shown in FIG. 11, devices of a belt heating type using an endless belt including an induction heat generating layer have been proposed. FIG. 12 shows an example of a conventional induction heating fixing device using an endless belt that is heated by electromagnetic induction (see, for example, JP10(1998)-74007 A).
In FIG. 12, reference numeral 960 denotes a coil assembly as an excitation unit that generates a high-frequency magnetic field. Reference numeral 910 denotes a metal sleeve (heat generating belt) that generates heat under a high-frequency magnetic field generated by the coil assembly 960. The metal sleeve 910 is formed by coating a surface of an endless tube made from a thin layer of nickel or stainless steel with a fluorocarbon resin. An inner pressing roller 920 is inserted in an inner portion of the metal sleeve 910, and an outer pressing roller 930 is placed outside the metal sleeve 910. The outer pressing roller 930 is pressed against the inner pressing roller 920 such that the metal sleeve 910 is interposed between them, and thus a nip part 950 is formed. While the metal sleeve 910, the inner pressing roller 920, and the outer pressing roller 930 rotate in the respective directions indicated by arrows, a high-frequency current is fed through the coil assembly 960. Thus, the metal sleeve 910 is heated rapidly by electromagnetic induction to a predetermined temperature. While predetermined heating is continued in this state, a recording material 940 is inserted into and passed through the nip part 950. Thus, a toner image formed on the recording material 940 is fixed on the recording material 940.
In the above-mentioned conventional induction heating fixing device of the roller heating type shown in FIG. 11, the following problems have been presented. That is, in the case of using a metallic material such as iron, aluminum, a stainless material or the like that is in common use for the core material 824 of the heat generating roller 820, the core material 824 itself generates heat by induction heating due to passing of the alternating magnetic fields H, resulting in a loss of power. Further, the heat generation of the core material 824 leads to the occurrence of problems such as, for example, damage to bearings supporting the core material 824 caused due to a high temperature.
Similarly, in the conventional induction heating fixing device of the belt heating type shown in FIG. 12, the following problems have been presented. That is, in the case where the inner pressing roller 920 is formed of a metallic material such as iron, aluminum, a stainless material or the like, a high-frequency magnetic field generated by the coil assembly 960 reaches the inner pressing roller 920, so that the inner pressing roller 920 generates heat, resulting in a loss of power. Further, the heat generation of the inner pressing roller 920 leads to the occurrence of problems such as, for example, damage to bearings supporting the inner pressing roller 920 caused due to a high temperature.